The invention relates to electric insulation for use especially in cables and wires for medium and high voltage from about a minimum of 10kV. The insulation is based upon polyolefin with an additive of 0.05 to 10% by weight (with a particle size of up to 50 .mu.m or an agglomerate size of up to 100 .mu.m) which retards formation of water trees.
In electrically stressed pololefin insulation, processes can occur which are called "electrochemical treeing" (ECT) or "water treeing". These processes, which are serious problems particularly in respect to the operating safety of plastic-insulated medium and high-voltage cables lead to the formation of tree-like structures, the so-called ECT structures.
The optical appearance of ECT structures which are visible with particularly high contrast and detail after suitable dyeing is manifold. Fundamentally, two forms can be distinguished:
"vented trees" which start from the surface of the insulation and extend into the insulation, and
"bow-tie trees" which are generated in the interior of the insulation.
The mechanism of the ECT formation has not been clarified to date. It is generally assumed, however, that an electric field and the presence of a polar liquid, especially water, are required for the formation of the ECT structures; the ECT structures are therefore also called water trees. The initiation points of the water trees always seem to be faults such as impurities, aggregate admixtures, voids, gaps, cracks or boundary surfaces, of which, however, only a part leads to the formation of water trees. The faults cannot be avoided completely in the case of insulation produced in a large technical scale. From these faults, the tree-like structures extend in the direction of the electric field.
Since ECT structures represent local changes of the insulating material, they can cause damage to the insulation especially with respect to the dielectric strength. Numerous attempts have therefore already been undertaken to prevent or at least retard the growth of water trees. In particular, additives to the insulation layer have been made.
Suitable additives with which the ECT formation can be prevented effectively and permanently are in particular:
barbituric acid and 2-thiobarbituric acid as well as derivatives thereof (DE-OS No. 32 02 828 or U.S. Pat. No. 4,458,044,
water-soluble alkali and earth-alkali phosphates as well as hydrolyzable phosphoric-acid esters (DE-OS No. 32 02 896 or U.S. Pat. No. 4,581,290,
substances with a certain particle or agglomerate size which are adsorption-active for heavy-metal ions or bind them in an ion exchange (DE-OS No. 33 18 988 or U.S. Pat. No. 4,623,755),
alcoholates of magnesium, calcium and aluminum (DE-OS No. 33 21 268 or U.S. Pat. No. 4,574,111),
potassium and sodium stannate as well as titanium oxysulfate (DE-OS No. 35 03 998), and
derivatives of pyrimidine and hexahydropyrimidine (DE-OS No. 35 16 971).
From DE-OS No. 33 18 988 or U.S. Pat. No. 4,623,755, cited above, it is known to use in a polyolefin base electric insulation as an additive for retarding water trees, a homogeneous distribution of 0.05 to 10% by weight of a substance which is adsorption-active for heavy metal ions or binds heavy metal ions in an ion exchange, the substance having a particle size of up to 50 .mu.m or an agglomerate size of up to 100 .mu.m. As additives can be used here aluminum oxide and oxide hydrates with a large active surface and/or aluminum silicates; preferably, however, the electric insulation contains pyrogenic and/or precipitated silicic acid. The additives are preferably synthetic products; however, minerals found in nature can also be used.
Although additives of the above-mentioned type have been found to be effective for retarding water trees, electric insulation, particularly cable and wire insulation, must exhibit a number of further important properties for practical employment. Among them are particularly thermally-oxidative stability, sufficient mechanical strength, low dielectric losses, optical transparency and storage stability as a processable insulating compound. Special attention must be given to the dielectric losses because they can be increased if additives are present in the insulation.
It is an object of the invention to further develop electric insulation of the type mentioned at the outset in such a manner that it meets the requirements of practice in every respect, ECT-retarding effectiveness and low dielectric losses being sought in particular.